Methods for preparing human thrombopoietin polypeptides by mammalian cell cultures

ABSTRACT

Procedures to produce a biologically active human TPO by mammalian cells in in vitro cultures are disclosed. Murine myeloma cells were genetically modified by introducing the gene of human thrombopoietin through a DNA construction which includes an immunoglobulin promoter associated to an immunoglobulin enhancer. The hTPO obtained is useful with methods of stimulating proliferation or development of hematopoietic cells of the megakaryocytic leneage in vitro and in vivo.

BACKGROUND OF THE INVENTION

In every normal adult, blood cells are produced at the bone marrow from stem cells through a process called “hematopoiesis”. After several steps of proliferation and differentiation, those progenitor pluripotent cells are capable of autoduplicating without differentiating themselves (endoduplication/autoreplication) or capable of generating a series of progenitor cells grouped in 4 principal ways of differentiation: a) erythroid (red blood cells or erythrocytes); b) myeloid (polimorphonuclear leucocytes and monocytes); c) lymphoid (lymphocytes) and d) megakaryiotic (generator of megakaryocytes/platelets). The quantity and cellular type produced by the bone marrow is a process finely regulated by a complex network of cytokines or growth factors that act via membrane-bound receptors on the target cells. These cytokines include a heterogeneous group of growth factors including interleukins (among others TNF-α, IL-3, IL-6, IL-8 e IL-11) and glycoproteic hormones called “Colony Stimulating Factors” (Granulocyte Colony stimulating factors and Granulocyte/Macrophage Colony stimulating factors [G-CSF y GM-CSF respectively]); Erythropoietin as stimulation factor of erythrocyte progenitors and Thrombopoietin as stimulator of platelet progenitors.

An increased knowledge of hematopoiesis and its regulation has lead to the use of diverse hematopoietic cytokines in medical treatment. In this way, medical treatments have been included based on:

-   -   Erythropoietin (EPO) for the treatment of secondary anemia in         chronic kidney failure.     -   Colony stimulating factors (G-CSF y GM-CSF) to accelerate the         recovery of the immune system in cancer patients undergoing         chemotherapy or those receiving a bone marrow transplant.     -   IL-2 and Interferon-α Both have been widely used in conjunction         with chemotherapeutic agents. Interferons are being used, with         certain success, in the treatment of chronic hepatitis, as well         as IL-2 in AIDS associated to antiretroviral agents.

The circulating blood platelets are produced in the bone marrow and play a crucial role in blood coagulation. Platelets adhere to sites of tissue damage and recruit others to aggregate with it to form the “primary hemostatic plug”. In addition, it serves as the surface upon which the coagulation factors are activated to produce a fibrin clot. In the absence of platelets, both of these functions are deficient and bleeding ensues (1). The generation of platelets implies proliferation and differentiation of bone marrow stem cells, into megakaryocytic cells which will generate the mature platelets (thrombocytes) that normally circulate in peripheral blood.

The physiological process by which platelets are generated requires the presence of factors called “Thrombocytopoietic” as growth and development factors from the megakaryocytic-lineage. In clinical medicine, the decrease of the number of platelets (thrombocytopenia) is a great complication related to two relevant situations: a) diseases that affect the normal generation of platelets (primary thrombocytopenia) and B) as a result of complications derived form therapeutic treatment in cancer patients or patients that have received a bone marrow transplant (secondary thrombocytopenia). The increased risk of hemorrhages in these patients actually requires the administration of platelet concentrates from normal donors with the associated biological risk (allergies, infections, immune hypersensitivity). From the early description in 1958, of a thrombopoietin activity in thrombocytopenic animals by Kelemen and cols. (2), we had to wait until 1994 in order to identify that factor, that was purified and cloned by several groups, as a glycoprotein capable of binding the cellular receptor Mpl (oncogene responsible of the murine myeloproliferative leukemia virus) (3) that was identified as Thrombopoietin (alternatively called: c-Mpl ligand; Megapoietin or Megakaryocyte Growth and Development Factor (MGDF)) able to specifically stimulate the megakaryocytopoiesis or thrombocytopoiesis (4-8). Other factors with thrombocytopoietic activity have been described (9), including IL-6 and IL-11 although they are not specific and have pleiothropic actions. The administration of high doses of IL-11 and IL-6 isolatedly, have resulted in low increments of circulating platelets (10) The gene of Human Thrombopoietin (hTPO) has been localized as a single copy, to chromosome 3 (3q26-27). It codifies for a protein of 353 amino acids (open reading frame of 10590 base pairs), including a signal peptide of 21 amino acids (aa) (63 base pairs). The mature protein has 332 aa (996 bp) and an estimated molecular weight of 38 kDa (not glycosilated). It has 6 N-glycosilation sites in order to generate a complete glycosilated protein of 70-80 kDa and a cleavage site, between arginines 153 and 154, where the protein is divided into two domains: a) a highly glycosilated c-terminal domain without homology with known proteins, and b) an EPO-like or N-terminal domain of 153 aa with 23% of identity with EPO. Allowing for conservative aa substitutions, sequence conservation approaches 50%. It is important to highlight that the non-glycosilated form has complete activity in vitro but not in vivo due to a decreased stability and a quick depuration in the circulation. Besides the native molecule (70-80 kDa), several forms of circulating TPO have been described, as a result of its partial proteolysis consisting of forms of 30,25 and 18 kDa, all truncated at the C-terminal domain (9, 11 and 12).

At present, the only factor with thrombocytopoietic activity available and approved by the FDA (Food and Drug Administration) is the human recombinant IL-11.

GENERAL DESCRIPTION OF THE INVENTION

From a general point of view this invention concerns:

Within one aspect, the present invention provides an expression vector replicable in mammalian host cells. The vectors comprise the following operably linked elements: 1) a transcription promoter; 2) the first DNA segment encoding a secretory leader or signal peptide of native hTPO; 3) a second DNA segment encoding the complete hTPO polypeptide; 4) a transcriptional enhancer; 5) polyadenylation signal and 6) a transcription terminator.

Within a second aspect of the invention, there is provided a cultured eukaryotic cell line containing the DNA construct as disclosed above. The prefered cell line is a mammalian cell.

Within a third aspect, there is provided a description of hTPO production procedure that has a murine cell line of plasmocytic origin, transfected with the espression vector previously described. Thus, this cell line acquires the capacity to synthesize and secrete a biologically active human recombinant hTPO (rhTPO), identical to its natural counterpart.

DETAILED DESCRIPTION OF THE INVENTION

Prior to describing the present invention and in order to facilitate its comprehension, it may be helpful to make a description summarizing and explaining some of the terms and procedures used in the present invention:

-   -   Thrombopoietin (TPO) It is a glycoprotein mainly produced by         hepatic and kidney cells in normal individuals. It is capable to         specifically bind to Mpl receptor from the same species         stimulating platelet production. Mpl receptor is mainly present         on pluripotent stem cells, megakaryocytes and platelets. The         prefix “h” makes reference to human TPO (hTPO). The term hTPO         encompasses full-length thrombopoietin molecules that exhibit         the qualitative biological activities of the intact molecule         (receptor binding and in vivo stimulation of platelet         production).     -   Recombinant Protein When a cell from a certain origin is         genetically modified, through recombinant DNA techniques, in         order to produce a protein that is not normally produced by it,         acquires the name of recombinant protein. Generally the prefix         “r” indicated that it is a recombinant biomolecule.     -   cDNA (Complementary Deoxyribonucleic Acid). Represents a         complementary copy of ribonucleic Acid (RNA) obtained by a         process called reverse transcription, by which a DNA polimerase         uses RNA as template (reverse transcriptase). The cDNA can be         single-stranded or double-stranded.     -   Expression Vector Is a DNA molecule, linear or circular, that         comprises a segment encoding a polypepetide of interest operably         linked to additional segments that provide for its         transcription. Such additional segments include promoter and         temrinator sequences. An expression vector may also include one         or more origins of replication, one or more selectable markers,         an enhancer, a polyadenylation signal, etc. Expression vectors         are generally derived from plasmid or viral DNA, or may contain         elements of both. The term “operably linked” indicates that the         segments are arranged so that they function in concert for their         intended purposes, e.g. transcription initiates in the promoter         and proceeds through the coding segment to the terminator.         Replication of expression vectors in a host organism can be         autonomous or through integration into the host genome.     -   Leader or signal sequence A DNA sequence encoding a secretory         peptide. Signal sequences are also called leader or prepro         sequences. A secretory peptide is an amino acid sequence that         acts to direct the secretion of a mature polypeptide or protein         from a cell. Secretory peptides are characterized by a core of         hydrophobic amino acids and are typically (but not exclusively)         found at the amino termini of newly synthesized proteins. Very         often the secretory peptide is cleaved from the mature protein         during secretion in one or more cleavage events. Such secretory         peptides contain processing sites that allow cleavage of the         secretory peptides from the mature proteins as they pass through         the secretory pathway.     -   Promoter DNA sequence recognized by RNA polimerase in order to         initiate the gene transcription and generate the corresponding         messenger RNA.     -   Transfection It is the process by which an eukaryotic cell is         genetically modified through the introduction of DNA         constructions that are capable of producing a protein that is         not normally produced by that cell. They are described as         “transfected cells”.

The present invention provides the material and methods used in order to obtain a human recombinant TPO with proved biological activity and whose general experimental design had the following steps:

-   1. Total RNA was isolated from normal human liver obtained from a     volunteer donor who had hepatobilliar surgery. -   2. Total cDNA was obtained from RNA by a process of reverse     transcription and the complete sequence of hTPO was amplified from     cDNA by using specific primers (Table II). -   3. We proceed to the construction of an expression vector containing     the specific cDNA of hTPO, in a form capable of being introduced in     an eukaryotic cell in order to induce the expression and production     of Human Thrombopoietin. -   4. That vector is introduced, by conventional electroporation     techniques, in a mammalian cell line that can be cultivated in     vitro. -   5. Neomycin antibiotic is added to the artificial medium in which     cells are grown. The cells that have successfully incorporated the     expression vector are selected because they bear the gene enconding     resistance to the antibiotic neomycin. -   6. By limited dilution methods, individual cells that host the     expression vector are selected. Cells producing a significant amount     of recombinant human Thrombopoietin are called “Clones”. -   7. Its biological activity is tested in in vitro and in vivo assays     using culture supernatants, by their ability to alternatively     induce: a) the differentiation of hematopoietic precursors towards     megakaryocytes in in vitro cultures; b) proliferation of     TPO-dependant cell lines; c) increase in circulating blood platelets     in thrombocytopenic animals (in vivo stimulation of platelet     production).

The nucleotide sequence identified as NM000460 and the aminoacidic sequence identified as SEQ N^(o)1 that corresponds to the mature hTPO is represented in Table I (5). The sequences NM 000460 and SEQ N^(o)1 represent a single allele of hTPO from which different allele variants may exist. Those variants can be obtained from cloned cells, human tissues or DNA preparations. That sequence was used to obtain primers necessary to amplify the specific TPO material and its posterior cloning and purification. In the present invention we proceed to clone and obtain the cDNA that corresponds to the complete hTPO including its signal peptide from normal hepatic tissue. No partial or modified fragments of the same sequence were used in the present invention.

The expression vector constructed in this invention (replicable in mammalian cells), as previously described, sequentially contains: a) a transcription promoter, derived from a variable gene from a kappa chain of human immunoglobulins obtained from a genomic library of human B lymphocytes; b) a first segment of DNA encoding the specific signal peptide of hTPO; c) a second segment of DNA encoding the full-length mature hTPO polypeptide; d) a transcriptional enhancer, derived from an intronic region from a human kappa gene (obtained from a genomic library of human B lymphocytes); e) a polyadenylation signal; f) a transcription terminator; g) the complete sequence of the resistance gene to ampicillin, as a selection marker in bacterial systems; h) the complete sequence of the resistance gene to neomycin, as a selection marker in eukaryotic cell systems; I) a bacterial replication origin that allows its expansion in bacteria. The direct product of transcription and transduction of the DNA segment that codes the mature hTPO polypeptide may depend on a post-transductional processing adding different chemical groups that include, among others, the union of carbohydrates through chemical bonds called “N-linked glycosilation or O-linked glycosilation”. The exact nature of this post-transductional modifications will be determined mostly but not exclusively, by the type of host cell used to produce recombinant hTPO.

Suitable host cells for use within the present invention include any type of eukaryotic cell that can be engineered to express heterologous DNA, can be grown in culture, and has an efficient secretory pathway. The preferred cells that follow these requirements are those derived from murine myeloma which are kown in the art and available from public depositories such as the American Type Culture Collection, Rockville, Md. (ATCC). In order to give a reference, the line X-63 (P63X63Ag.563 (ATCC N^(o) CRL-1580) and SP2 (ATCC N^(o) CRL-1581 and CRL-8287). In order to direct a hTPO plypeptide into the secretory pathway of the host cell, a DNA sequence encoding a secretory leader is used in combination with a DNA sequence encoding a hTPO polypeptide. In the present invention the signal peptide used was the corresponding to native hTPO (aminoacidic sequence appears in bolded font over SEQ 1 in Table I). Generally strong promoters of transcription are required, such as immunglobulins, viral promoters derived from SV40, adenovirus or citomegalovirus that are included here in order to have a reference.

Drug selection is a widely used method that allows selecting those mammalian culture cells that have successfully incorporated the expression vector. Such cells are commonly referred to as “transfectants”. Cells that have been cultured in the presence of the selective agent and are able to pass the gene of interest to their progeny are referred to as “stable transfectants”. A preferred selectable marker is a gene encoding resistance to the antibiotic neomycin. Selection is carried out in the presence of neomycin-type drug, such as G-418 or the like. Selection systems may also be used to increase the expression level of the gene of interest, a process referred to as “amplification”. Amplification is carried out by culturing transfectants in the presence of a low level of the selective agent and then increasing the amount of selective agent to select for cells that produce high levels of the products of the introduced genes. Other drug resistance genes that can be used may be the hygromycine, multi-drug resistance gene, dihydrofolate reductase and puromycin acetlyltransferase.

There are many methods to incorporate exogenous DNA into mammalian cells, among which we highlight: a) calcium phosphate-mediated transfection; b) electroporation (14); c) DEAE-dextrane mediated transfection (Ausubel et al., eds. Current Protocols in Molecular Biology, John Wiley and Sons, Inc., NY, 1987) and d) cationic lipid-mediated transfection. The preferred method used in the present invention is electroporation. The transfected cells are cultured in conventional media, which contain essential nutrients for cellular growth, as well as the necessary antibiotics in order to select the cells that have incorporated exogenous DNA.

The hTPO polypeptide prepared according to the present invention can be used therapeutically wherever it is desirable to increase platelet production, such as in the treatment of thrombocytopenia induced by different diseases including aplastic anemia, myelodisplastic syndromes, chemotherapy or congenital thrombocytopenias.

The thrombocytopenia is characterized by a decrease in the number of circulating blood platelets and is manifested as increased skin and mucosal bleedings. Lowered platelet counts can result from, for example, defects in platelet production, abnormal platelet distribution, dilutional losses due to massive transfusions, or abnormal destruction of platelets. In addition, certain malignancies can impair platelet production and platelet distribution. Radiation therapy used to kill malignant cells also kills platelet progenitor cells. Thrombocytopenia may also arise from various platelet autoimmune disorders induced by drugs, neonatal alloimmunity or platelet transfusion alloimmunity. hTPO polypeptides can reduce or eliminate the need for transfusion, thereby reducing the incidence of platelet alloimmunity. Abnormal destruction of platelets can result from: 1) increased platelet consumption in vascular grafts or traumatized tissue; or 2) immune mechanisms associated with, for example, drug-induced thromocytopenia, idiopathic thrombocytopenic purpura (ITP), autoimmune diseases, hematologic disorders such as leukemia and lymphoma, or metastatic cancers involving bone marrow. Other indications for hTPO include aplastic anemia and rug-induced marrow suppression resulting from, for example, chemotherapy or treatment of HIV infection with AZT.

rhTPO can be also associated with other cytokines such as SCF, IL-3, IL-6, IL-11 or GM-CSF. The therapeutic doses of rhTPO are in the range of 0.1 a 100 μg/kg per day (preferably 0.5-50 μg/kg a day). The exact doses must be determined by clinical evaluation in each particular case. The rhTPO is administered for a 28 day period following chemotherapy or bone marrow transplant, or until the number of platelets is more than 20.000/μl, preferably 50.000/μl.

rhTPO polypeptides are also valuable tools for the in vitro study of the differentiation and development of hematopoietic cells, such as for elucidating the mechanisms of cell differentiation and for determining the lineages of mature cells, and may also find utility as a proliferative agent in cell culture.

rhTPO polypeptides can also be used ex vivo, such as in autologous marrow culture. Briefly, bone marrow is removed from a patient prior to chemotherapy and treated with rhTPO polypeptides, optionally in combination with one or more other cytokines. The treated marrow is then returned to the patient after chemotherapy to speed the recovery fo the marrow. In addition, rhTPO polypeptides can also be used for the ex vivo expansion of marrow or peripheral blood progenitor (PBPC) cells. Prior to chemotherapy treatment, marrow can be stimulated with stem cell factor (SCF) or G-CSF to release early progenitor cells into peripheral circulation. These progenitors can be collected and concentrated from peripheral blood and then treated in culture with one or more rhTPO polypeptides, optionally in combination with one or more other cytokines, cinluding but not limited to SCF, G-CSF, IL-3, GM-CSF, IL-6 or IL-11, to differentiate and proliferate into high-density megakaryocyte cultures, which can then be retured to the patient following high-dose chemotherapy.

Another potential use of rhTPO, is its use in several diagnostics that are performed in order to determine the presence of anti-TPO antibodies, which have been demonstrated in autoimmune thrombocytopenias.

The present invention is futher illustrated by the following non-limiting examples.

EXAMPLES Example 1

mRNA obtained from hepatic tissue of a normal adult in order to obtain specific cDNA of hTPO. A fragment of hepatic tissue from a normal adult (50 mgrs) was processed in order to isolate total RNA. This process was performed applying classical methods of extraction with guanidine thiocyanate phenol-chloroform. The purity and concentration of total RNA was determined by 260/280 absorbance ratio.

Example 2

cDNA synthesis (reverse transcription) of hTPO specific mRNA. Synthesis of the first stand of cDNA by reverse transcription, was performed from 5 ug of total RNA, using synthetic Oligo(dT)₁₂₋₁₈ and Reverse Transcriptase (SUPERSCRIPT™ II, Gibco-BRL-Life Technologies). The assay was performed in a total volume of 20 μl containing: 4 μl buffer 5×(250 mM Tris-HCl pH 8.3; 350 mM KCl; 15 mM MgCl₂; 0.1 M DTT); 1 μl of Oligo (dT) 500 μg/ml; 2 μl DTT 0.1 M.; 1 μl 10 mM dNTPs; 5 μg total ARN and completing with 20 μl H₂O.

Example 3

Amplification of the specific CDNA encoding the complete hTPO. Amplification of the sequence of interest by “nested PCR” techniques (Polymerase chain reaction). This is done by designing a double pair of primers and a combination of thermostable DNA polymerases in order to amplify the sequence encoding the mature protein that has high specificity and fidelity. In that sense, a high fidelity amplification system was used. This system was composed by the adequate mixture of Taq polymerase and Pow DNA polymerase (EHF; Expand™ High Fidelity PCR System, Boehringer Mannheim). The sequence of primers used in the two amplification rounds are described in Table II. The amplification reactions were performed in a total volume of 100 μl by adding:

-   -   2 μl (dNTPs 200 □M)     -   3 μl (10 mM forward primer)     -   3 μl (10 mM reverse primer)     -   10 μl (buffer 10×)     -   0.75 μl (Thermostable polymerases EHF)     -   5 μl (cDNA)     -   76.25 μl (H₂O)

The first round of amplification was performed with the external primers TPO_(EXT-FOW) y TPO_(EXT-REV) (see Table II A) and the reaction was then incubated for 30-amplification cycle (previously incubated for 2 minutes at 95° C.).

-   -   Cycles 1-10: 30 sec. at 95° C. 30 sec. at 58° C. and 120 sec. at         72° C.     -   Cycles 10-20: 30 sec. at 95° C. 30 sec. at 58° C. and 240 sec.         at 72° C.     -   Cycles 20-30: 30 sec. at 95° C. 30 sec. at 58° C. and 360 sec.         at 72° C.     -   Final elongation stage: 10 minutes at 72° C.

The second round of amplification was performed with the internal primers TPO_(FOW) y TPO_(REV) (see Table II B) under the same conditions, but substituting 5 μl of cDNA of the first round for 2 μl of the first amplification reaction. The reaction conditions were 20 cycle of amplification with previous incubation for 2 minutes at 95° C., described as follows:

-   -   Cycles 1-10: 30 sec. at 95° C. 30 sec. at 62° C. and 120 sec. at         72° C.     -   Cycles 10-15: 30 sec. at 95° C. 30 sec. at 62° C. and 240 sec.         at 72° C.     -   Cycles 15-20: 30 sec. at 95° C. 30 sec. at 62° C. and 360 sec.         at 72° C.     -   Final elongation stage: 10 minutes at 72° C.

Example 4

Purification and preparation of amplified cDNA of hTPO in order to be cloned. The amplified product is separated and identified by electrophoresis, in an agarose gel of low fusion point (1%). The part of the gel containing the band of 1107 base pairs was cut and the DNA was isolated and purified (Nucleiclean, Sigma). This was carried out performing techniques that use the capacity of fiber-glass microspheres in order to strongly bind the DNA in a saturated sodium iodide media which at the same time is capable of dissolving the agarose (16). The DNA is extracted with phenol/chloroform, ethanol washed, precipitated with ethanol, being finally resuspended in 10 ul of H₂O. In order to incorporate adenine nucleotides at the 3′OH extremes, for cloning in A-T vectors, the purified fragment is incubated for 10 minutes at 72° C. with: Taq DNA polymerase, buffer and dATP. The fragment is again extracted and purified using phenol/chroloform extraction, precipitated in ethanol and then resuspended in 20 μl of H₂O. The cDNA fragment is ready to be ligated and cloned into an A-T cloning vector system.

Example 5

Cloning of amplified cDNA fragment into an A-T cloning vector system. The purified fragment of experiment 4 was ligated to pCR 2.1 plasmid (Original TA Cloning Kit; Invitrogen). The ligation was performed using T4 DNA ligase (Boehringer Mannheim), pCR2 vector and the purified cDNA hTPO, in a saline medium according to the following:

-   -   2 μl of TPO fragment purified in Example 4.     -   1 μl of ligation buffer 10×     -   2 μl of pCR2.1 vector (25 ng/μl)     -   1 μl of T4 DNA ligase (4 units)     -   4 μl of H₂O

The enzymatic ligation was incubated for 16 hours at 14° C.

Competent E. Coli strains (ONE SHOT™ TOP10F′; Invitrogen) were transformed with the ligated plasmid. This vectors allowed: a) obtaining bacterial colonies that included the plasmid, due to the presence of the resistance gene to ampicillin; b) identifying bacterial colonies with plasmids containing cDNA inserts encoding hTPO. This is performed depending on the presence of the lacZα□gene in the vector, which is inducible by IPTG isopropylthio-β-galactoside), allowing to trace them, identifying the bacterial colonies white/blue according to X-Gal hydrolysis; c) isolation of high quantities of plasmids (minipreps) from positive bacterial colonies by minicultures in LB (Luria-Bertani) in order to perform restriction analysis and sequenciation of the inserts using M13 primers in order to verify the sequence. After restriction analysis 10 positive colonies are obtained, correlatively called pCR2 TPO-1 to pCR2 TPO-10.

The isolated colonies were segregated into three groups:

-   -   Clones pCR2-1, -2, -4 and -9 matching the corresponding sequence         of hTPO, where 12 bases were deleted (4aa), identical to the         previously described isoform called TPO2 (17).     -   Clones pCR2-5, -6, -7, -8 and -10 matching a complete sequence         of native hTPO, with no aminoacidic substitution.     -   Clone pCR2-9 matches the corresponding sequence of hTPO, with a         deletion of 116 bp identical to the previously described isoform         TPO3 (17).

The inserts were sequenced, in order to be correctly identified, according to the method described by Sanger and cols. (18) The sequenciation was performed completely over both DNA strands.

The plasmid preparation corresponding to clone pCR2 TPO-5 was selected to continue the following experimental steps.

Example 6

Vector design with the sequence encoding mature hTPO (pK-eTPO). An eukaryotic expression vector was constructed, according to the following characteristics:

-   -   1. Neomycin-resistance gene (Neo^(r)) to positively select the         transfected cells in a medium that contains neomycin (G-418)     -   2. Promoter sequence for immunoglobulins genes     -   3. A cloned region containing the restriction sites Cla I and         Not I     -   4. Enhancer of human immunoglobulin transcription     -   5. Prokaryotic replication origin for E. Coli (ColE1 ori)     -   6. Ampicillin-resistance gene (Amp^(r))     -   7. Eukaryotic replication origin derived from SV 40 (SV 40 ori)

All these sectors are logically ordered to assure transcription and transduction of the corresponding rhTPO gene. The construction of the expression vector was identified as pK and can be graphically observed in FIG. 1.

Example 7

Construction of an eukaryotic expression vector using the sequence encoding mature hTPO (pK-eTPO) The construction of the pK expression vector that has the DNA fragment encoding hTPO and whose sequence was totally verified was called pK-eTPO. A graphic map of pK-eTPO structure, including all functional segments is depicted in FIG. 1. The construction was performed in the following experimental stages:

Stage 1

First, expression primers containing the Cla I and Not I restriction sites were designed (see Table II C), and were called Cla-eTPO_(FOW) and Not-eTPO_(REV). They allow to:

-   -   1) Amplify the sequence that codes hTPO that is contained in the         cloning vector obtained from pCR2 TPO-5.     -   2) Introduce two restriction sited (Cla I and Not I in order to         introduce the complete sequence encoding hTPO including its         signal peptide in pK vector after an enzymatic ligation         reaction.

The amplified hTPO fragment that is contained in pCR2 TPO-5, and that was modified to contain the Cla I and Not I restriction sites, was called eTPO-5. The amplification reaction was performed in a total volume of 50 μl (Taq DNA polymerase kit; Gibco BRL-Life Technologies):

-   -   2.5 RI (dNTPs 10 mM)     -   2.5 μl (10 mM of Cla-eTPO_(FOW))     -   2.5 μl (10 mM of Not-eTPO_(REV))     -   5 μl (buffer 10×)     -   0.5 μl (Taq polymerase)     -   5 μl (dilution 1/100.000 of pCR2 TPO-5)     -   1.5 μl MgCl₂ (50 mM)     -   30.5 μl (H₂O)

Incubation conditions: one 4-minute cycle at 95° C.; 25 cycles at 95° C. 30″, 65° C. 30″, 72° C. 30″; and one final elongation cycle of 10 min. at 72° C.

Stage 2

The amplified product is separated and identified using electrophoresis techniques in an agarose gel low fusion (1%). The part of the gel containing the band of 1090 bp was cut and the DNA was isolated and purified (Nucleiclean, Sigma). This was carried out by performing techniques that use the capacity of fiber-glass microspheres in order to strongly bind the DNA in a saturated sodium iodide media which at the same time is capable of dissolving the agarose (16). Then the DNA is extracted with phenol/chloroform and washed, precipitated with ethanol, being finally resuspended in 15 μl of H₂O.

Stage 3

The purified eTPO fragment is ligated into the pCR2-1 vector in order to perform enzymatic restriction analysis and nucleotide sequenciation to verify the complete identity of the fragment with the native hTPO according to the following reaction scheme:

-   -   1 μl eTPO fragment.     -   1 μl ligation buffer 10×     -   2 μl pCR2.1 vector (25 ng/μl)     -   1 μl T4 DNA ligase (4 units)     -   5 μl H₂O

The reaction is incubated for 16 hours at 4° C. The ligation product is used to transform competent E. Coli strains (ONE SHOT™ TOP10F′; Invitrogen) to allow their expansion according to the vendor's technical specifications. Six bacterial colonies containing the correct plasmid bearing eTPO inserts were identified and isolated. They were called pCR2-eTPO 1 to 6. Minicultures in LB medium with ampicillin were performed in each of them, in order to isolate the plasmidic material (minipreps).

Stage 4

Inserts eTPO-1 to 6 were identified and sequenced. This was done using the BamH I restriction profile and then sequenced to verify the complete nucleotide sequence in both DNA strands using the method described by Sanger and cols. (18) in an automated sequenciator (ALF-EXPRESS; Pharmacia). Table I shows the nucleotide sequence identified as “ETPO”, that corresponds to pCR2-eTP02 plasmid, compared to sequence NM000460 (Sauvage et al., 1994). As depicted, the obtained sequence is identical to the one used as reference. The pCR2-eTPO2 plasmid was selected to carry out the subsequent stages.

Stage 5

The eukaryotic pK expression vector was enzymatically digested using Cla I and Not I before it was ligated to the eTPO fragment. To perform the digestion, the enzymatic reaction was incubated for 4 hours at 37° C. as follows:

-   -   10.0 μl of vector pK plasmid preparation     -   2.0 μl of Cla I enzymatic preparation (Sigma)     -   1.5 μl of Not I enzymatic preparation (Sigma)     -   5.0 μl of buffer 10×     -   31.5 μl of H₂O

The linearized vector, after enzymatic digestion, is separated and identified using electrophoresis techniques with low fusion agarose gel (1%). The region, in which the vector was contained, was cut and DNA was isolated and purified by electroelution in dialysis membranes. The DNA is extracted with phenol/chloroform, washed, and ethanol precipitated, being finally resuspended in 15 μl H₂O.

Stage 6

The pCR2-eTPO2 was enzymatically digested using Cla I and Not I in order to obtain the eTPO-2 fragment. For the digestion the following enzymatic reaction was used which was incubated for 4 hours at 37° C.:

-   -   5.0 μl of plasmidic preparation of pCR2-eTPO 2     -   2.0 μl of enzymatic preparation of Cla I (Sigma)     -   1.5 μl of enzymatic preparation of Not I (Sigma)     -   5.0 μl of buffer 10×     -   36.5 μl of H₂O

The linearized vector, after enzymatic digestion, is separated from the eTPO 2 fragment using electrophoresis techniques with low fusion agarose gel (1%). The gel region in which the fragment eTPO 2 is contained was cut and DNA was isolated and purified by techniques that use the capacity of fiber-glass microspheres in order to strongly bind the DNA in a saturated sodium iodide media which at the same time is capable of dissolving the agarose (16). The DNA is extracted with phenol/chloroform, washed, precipitated with ethanol, being finally resuspended in 15 μl of H₂O.

Stage 7

The ligation of pK vector with eTPO-2 purified fragment is performed using T4 DNA ligase (Pharmacia) following the supplier's technical specifications. The product of this ligation, called pK-eTPO, is purified using phenol/chloroform extraction methods, precipitated with ethanol, being finally resuspended in 15 μl H₂O.

Stage 8

Transfection of E. Coli (strain XL1), with the constructed pK-eTPO is performed by electroporation. In a 1 mm cuvette, 40 ul of electro-competents XL-1 strain are placed together with 2 μl of the purified ligation obtained in stage 7. A 1380 v pulse with 129 Ω of resistance, capacitance of 50 μF, was triggered for 5 milliseconds. After the pulse, 800 μl of LB medium are added. After an incubation period of 45 min. at 37° C., in an orbital incubator at 200 rpm, LB-agar-ampicillin dishes are seeded.

Two positive colonies are isolated, and expanded in LB-ampicillin overnight minicultures. Plasmids were extracted using conventional methods (“minipreps”) in order to verify the inserts, by amplification and restriction enzyme analysis. In this way a purified plasmid fraction of pK-eTPO was obtained.

Example 8

pKeTPO vector linearization with Puv I and selection of the mammalian cell line to be transfected. Once pK-eTPO is obtained, the mammalian cell line derived from murine myeloma known as X-63 (P3X63Ag8.653, ATCC N^(o) CRL 1580), was selected, although any cell that is derived from a mammal can be eventually used with this expression plasmid. Among the different cell lines, this one was selected as it has the following advantages: a) it has a high-efficacy synthesis and secretory system of proteins (immunoglobulins) that perfectly adapt to the designed expression vector, in the present invention, as it includes a promoter of immunoglobulins genes associated to an immunoglobulin transcription enhancer. Additionally is capable of undefinitively grow in in vitro culture media such as RPMI-1640 supplemented with fetal bovine serum (FBS) 10%. pKeTPO vector was linearized with Pvu I (Boehringer Mannheim), according to the following reaction:

-   -   20 μl of plasmid pKeTPO fraction     -   5 μl of buffer H (10×)     -   2 μl of Pvu I     -   23 μl of H₂O

The reaction was incubated at 37° C. during 4 hours and the material was purified using phenol/chloroform extraction, precipitated with ethanol and finally recovered in 10 μl of H₂O.

Example 9

Transfection of X-63 cells with pKeTPO vector. The mammalian cells (X-63) described in example 8 were expanded in vitro in RPMI-1640 (Gibco BRL-Life Technologies) with 10% FBS, then washed in RPMI-1640 free of FBS. They were resuspended in the following concentrations: 1.5 million of cells in 800 μl RPMI 1640 without FBS. Then, the 800 μl of cells were placed in a 2 mm electroporation cuvette, adding 10 μg of pKeTPO plasmid. The electroporation (BTX Electroporation System 600; Genetronics Biomedical LTD) was performed at 13 msec under the following conditions:

-   -   C: 1200 μF; R: 48 Ω; V: 210 V; E: 1.050 kV/cm

The electroporated cells were cultured for 24 hours in RPMI 1640 with 20% FBS at 37° C. and 5% CO₂.

Example 10

Screening and selection of cloned cells producing rhTPO. After transfecting the cells, described in example 9, they were distributed in 3 different culture groups (culture I, II and III) with neomycin (1 mg/ml). The base culture medium was RPMI 1640 with 10% FBS and streptomycin-penicillin (50 mcg/ml and 100 UI/ml respectively), 2 mM L-glutamine 1 uM sodium piruvate and 10 mM Hepes (growth medium). After 15 days of breeding, with a weekly change of culture media, the secretion of rhTPO in supernatants was analyzed performing western-blot using anti-hTPO antibody (RDI; Research Diagnostics, Inc.). As negative controls, supernatants of X-63 transfected cell with pK kappa vector, were used. The proceedures were identical to those of TPO including the kappa human chain of immunoglobulin instead of the rhTPO insert.

Supernatants from culture II resulted weakly positive for rhTPO, and cloning was performed by limit dilution (cell dilution: 0.3 cells/well placed in a 96-well plates in RPMI1640 with 20% of FBS and 1 mg/ml neomycin). After 15 days the 58 neomycin-resistant clones were isolated. Supernatants were processed to evaluate rhTPO presence by western blot. The analysis revealed the presence of 16 clones that produced and secret rhTPO. The clone called 2A4 was isolated, as significant quantities of rhTPO were detected in culture supernatants and was again cloned, by limit dilution, in order to assure its clonaility.

As it is shown in FIG. 2, after analysis by western-blot, subclone 16 from 2A4 was selected, being called subclone 24A-16. The production stability of rhTPO using sub clone 24A-16 was tested after 8 weeks of uninterrupted culture (see FIG. 2). The clone from X-63 cell called 24A-16 and producer of soluble rhTPO in a stable way, will be called Clone X-63 eTPO.

Example 11

Analysis of the presence and biological activity of rhTPO produced in this expression system. Assays testing biological activity were performed in vitro, using the supernatant of X-63eTPO culture at concentrations of 0.6×10⁶ cells/culture in presence of RPMI 1640, 10% FBS:

-   -   Determination of TPO concentration by ELISA. An ELISA test was         performed using a kit for human TPO (Quantikine, R&D Systems).         It was confirmed that approximate concentration, under standard         culture conditions, was 0.5 mcg/ml.     -   In vitro bioassay using the Baf-mpl cell line. The Baf-mpl cell         line (deposited Sep. 28, 1994 under the Budapest treaty terms in         ATCC N^(o) CRL-11723) corresponds to the murine line Baf-BO3         transfected with the Mpl receptor (TPO receptor). The parental         line Baf-BO3 depends on IL-3 and transfected line Baf-mpl         depends on IL-3 or TPO. The assay is based on incubating the         cells in the presence of different concentrations of cytokines         or the supernatant that is being tested during 48 hours. After         this period they are incubated with [3_(H)]-Thymidine for twelve         hours after which the cells are recovered over a filter and then         placed in vials where radioactivity is tested by liquid         scintillation counting. Two experiments were performed in         96-well plates testing supernatants at different concentrations:         10%, 3%, 1%, 0.3%, 0.1%, 0.03%, 0.01% and 0.003%, in triplicate         over Baf-mpl cells (30.000 cells/well). A dose-dependant curve         was performed with standard TPO at 10 ng/ml, 3 ng/ml, 1 ng/ml,         0.3 ng/ml, 0.1 ng/ml, 0.03 ng/ml, 0.01 ng/ml and 0.003 ng/ml         concentrations. Internal control test with IL-3 was performed in         order to assure that the cell line responds to the analyzed         cytokines. A negative control was performed without growth         factors. The same was performed with the parental line Baf-BO3         in order to discard an IL-3 effect in the tested supernatants.         Experiments showed similar results indicating a rhTPO standard         activity of 0.4 mcg/ml in supernatants culture of X-63eTPO.     -   In vitro proliferation and differentiation of megakaryocytic         progenitors from CD34+ bone marrow cells. The bone marrow cells         CD34+ were obtained through gradient density centrifugation with         Ficoll-Hypaque and then immunomagnetically selected (Miltenyi,         MiniMACS). These cells are cultured in liquid medium under         conditions that allow differentiation towards the megakaryocytic         lineage. These conditions are: FBS-free Iscove culture media         supplemented with transferrine, insulin, bovine seroalbumin,         liposomes and in the presence of TPO. CD34+ bone marrow cells         were cultured in 24-well plates (100.000 cells/well) with         different dilutions of standard rhTPO or supernatant of X-63eTPO         clone. After 10 days cell proliferation is tested (number of         cells) and percentage of megakaryocytic produced by marking them         with FITC-conjugated anti-CD41 monoclonal antibodies and         analyzed by flow citometry. Four different concentrations of         rhTPO were tested (20 ng/ml, 10 ng/ml, 5 ng/ml, 2.5 ng/ml). It         was observed that 70% of the cells were of megakaryocytic         lineage (CD41 positive cells). Supernatants from X-63eTPO were         tested at 3 different concentrations: 10%, 5% y 2.5%. An         increase of 3, 2 and 1.5 times, in cell numbers were         respectively observed. Regarding megakaryocytic growth we         obtained 75% of CD41 + cells, in the three cases, so the         absolute number of megakaryocytes varies according to         supematant's concentration being related to cell proliferation.         Representative results analyzed by flow citometry are shown in         FIG. 3, where it can be appreciated that 2.5% supematant culture         is capable of inducing an expression level equivalent to that         induced by 5 ng/ml of standard hTPO.

TABLE I Reference nucleotidic and aminoacidic sequence of the hTPO NM 000460 [de Sauvage et al, Nature 369; 533–538 (1994)]. SEQ N^(o)1                    M  E  L  T  E  L  L  L  V  V  M  L  L  L  T  A NM 000460 CATATCGATTTCTCACAATGGAGCTGACTGAATTGCTCCTCGTGGTCATGCTTCTCCTAACTGCA ETPO                   ................................................                   -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 R  L  T  L  S  S  P  A  P  P  A  C  D  L  R  V  L  S  K  L  L  R NM 000460 AGGCTAACGCTGTCCAGCCCGGCTCCTCCTGCTTGTGACCTCCGAGTCCTCAGTAAACTGCTTCGT ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 D  S  H  V  L  H  S  R  L  S  Q  C  P  E  V  H  P  L  P  T  P  V NM 000460 GACTCCCATGTCCTTCACAGCAGACTGAGCCAGTGCCCAGAGGTTCACCCTTTGCCTACACCTGTC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 L  L  P  A  V  D  F  S  L  G  E  W  K  T  Q  M  E  E  T  K  A  Q NM 000460 CTGCTGCCTGCTGTGGACTTTAGCTTGGGAGAATGGAAAACCCAGATGGAGGAGACCAAGGCACAG ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 D  I  L  G  A  V  T  L  L  L  E  G  V  M  A  A  R  G  Q  L  G  P NM 000460 GACATTCTGGGAGCAGTGACCCTTCTGCTGGAGGGAGTGATGGCAGCACGGGGACAACTGGGACCC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 T  C  L  S  S  L  L  G  Q  L  S  G  Q  V  R  L  L  L  G  A  L  Q NM 000460 ACTTGCCTCTCATCCCTCCTGGGGCAGCTTTCTGGACAGGTCCGTCTCCTCCTTGGGGCCCTGCAG ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 S  L  L  G  T  Q  L  P  P  Q  G  R  T  T  A  H  K  D  P  N  A  I NM 000460 AGCCTCCTTGGAACCCAGCTTCCTCCACAGGGCAGGACCACAGCTCACAAGGATCCCAATGCCATC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 F  L  S  F  Q  H  L  L  R  G  K  V  R  F  L  M  L  V  G  G  S  T NM 000460 TTCCTGAGCTTCCAACACCTGCTCCGAGGAAAGGTGCGTTTCCTGATGCTTGTAGGAGGGTCCACC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 L  C  V  R  R  A  P  P  T  T  A  V  P  S  R  T  S  L  V  L  T  L NM 000460 CTCTGCGTCAGGCGGGCCCCACCCACCACAGCTGTCCCCAGCAGAACCTCTCTAGTCCTCACACTG ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 N  E  L  P  N  R  T  S  G  L  L  E  T  N  F  T  A  S  A  R  T  T NM 000460 AACGAGCTCCCAAACAGGACTTCTGGATTGTTGGAGACAAACTTCACTGCCTCAGCCAGAACTACT ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 G  S  G  L  L  K  W  Q  Q  G  F  R  A  K  I  P  G  L  L  N  Q  T NM 000460 GGCTCTGGGCTTCTGAAGTGGCAGCAGGGATTCAGAGCCAAGATTCCTGGTCTGCTGAACCAAACC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 S  R  S  L  D  Q  I  P  G  Y  L  N  R  I  H  E  L  L  N  G  T  R NM 000460 TCCAGGTCCCTGGACCAAATCCCCGGATACCTGAACAGGATACACGAACTCTTGAATGGAACTCGT ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 G  L  F  P  G  P  S  R  R  T  L  G  A  P  D  I  S  S  G  T  S  D NM 000460 GGACTCTTTCCTGGACCCTCACGCAGGACCCTAGGAGCCCCGGACATTTCCTCAGGAACATCAGAC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 T  G  S  L  P  P  N  L  Q  P  G  Y  S  P  S  P  T  H  P  P  T  G NM 000460 ACAGGCTCCCTGCCACCCAACCTCCAGCCTGGATATTCTCCTTCCCCAACCCATCCTCCTACTGGA ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 Q  Y  T  L  F  P  L  P  P  T  L  P  T  P  V  V  Q  L  H  P  L  L NM 000460 CAGTATACGCTCTTCCCTCTTCCACCCACCTTGCCCACCCCTGTGGTCCAGCTCCACCCCCTGCTT ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 P  D  P  S  A  P  T  P  T  P  T  S  P  L  L  N  T  S  Y  T  H  S NM 000460 CCTGACCCTTCTGCTCCAACGCCCACCCCTACCAGCCCTCTTCTAAACACATCCTACACCCACTCC ETPO .................................................................. -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  - SEQ N^(o)1 Q  N  L  S  Q  E  G  # NM 000460 CAGAATCTGTCTCAGGAAGGGTAAGCGGCCGCTCT ETPO ................................... -  -  -  -  -  -  -  - (.) identical nucleotidic sequence (-) identidad aminoacidic sequence (#) stop codon ETPO represent de sequence of hTPO obtained in the present invention Secretory peptide is depicted in bold characters. Internal primers are represented in underlined characters

TABLE II Sequence of primers used to amplify human TPO A. Primers used in the first round of the nested PCR. TPO _(EXT-FOW). GAG GAA GGA TTC AGG GGA GAG G (SEQ ID NO:3) (External forward primer) TPO _(EXT-REV). GGA AGG GAG CTG TAC ATG AGA C (SEQ ID NO:4) (External reverse primer) B. Primers used in the second round of the nested PCR. TPO _(FOW). AGC CAC GCC AGC CAG ACA CCC C (SEQ ID NO:5) (Internal forward primer) TPO _(REV). GCA GTG TCT GAG AAC CTT ACC C (SEQ ID NO:6) (Internal reverse primer) C. Primers used to insert the restriction sites ClaI y Not I into the DNA fragment encoding hTPO. Cla-eTPO _(FOW). CAT ATC GAT TTC TCA CAA TGG AGC TGA CTG AAT TGC TCC (SEQ ID NO:7) Not-eTPO _(REV). AGA GCG GCC GCT TAC CCT TCC TGA GAC AGA TTC TGG G   (SEQ ID NO:8)

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1. An expression vector replicable in mammalian cells, comprising the following elements grouped in a way that assures a correct and efficient transcription of a fragment comprising the complete sequence encoding the human thrombopoictin (hTPO) polypeptide including its signal peptide (eTPO): (i) a hTPO transcription portion consisting of: a) a transcription promoter of a variable kappa gene of human immunoglobulins b) a DNA segment encoding the complete hTPO polypeptide including its signal peptide c) an initnunoglobulin transcription enhancer DNA segment of the human kappa gene in a 3′ location with respect to hTPO gene, wherein said enhancer DNA segment consists of the DNA sequence 5′ to the Ck region of the human kappa gene d) a transcription terminator e) a polyadenylation signal; and (ii) gene encoding resistance to the antibiotic neomicyn type drugs (iii) a gene encoding resistance to the antibiotic ampicillin; and (iv) a replication origin for E Coli (ColEI ori).
 2. A cultured cell line derived from the munine myeloma called X-63 (ATCC: P63X63Ag8.6533, CRL 1580), which has been stably transfected by the expression vector described in claim 1, and tat is capable of producing and secreting the mature recombinant human thrombopoietin (rhTPO) polypeptide towards the culture media in a biologically active form.
 3. A method for obtaining a recombinant polypeptide corresponding to human thrombopoietin (hTPO), comprising the steps of: (a) stably transfecting a cultured cell line derived from the murine myeloma called X-63 (ATCC; P63X63Ag8.6533, CRL 1580) with the expression vector described in claim 1, and (b) expressing the mature recombinant human thrornbopoietin polypeptide. 